An electrorheological fluid (i.e., a fluid having an electroviscous effect) is known as a suspension of inorganic or polymeric particles in an electrically insulating liquid, which changes its viscosity rapidly and reversibly from a liquid state to a plastic state or a solid state or vice versa on application of an electric field. This phenomenon is called a Winslow effect.
Dispersion particles whose surface is readily depolarizable under an electric field are usually used in an electrorheological fluid. For example, inorganic dispersion particles include silica as disclosed in U.S. Pat. No. 3,047,507, British Patent 1,076,754 and JP-A-61-44998 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and zeolite as disclosed in JP-A-62-95397. Polymeric dispersion particles include alginic acid, glucose having carboxyl groups, and glucose having sulfone groups as disclosed in JP-A-51-33783, polyacrylic acid crosslinked with divinylbenzene as disclosed in JP-A-53-93186, and a resol type phenolic resin as disclosed in JP-A-58-179259.
The electrically insulating fluid includes hydrocarbon-based oils, silicone oils, ester-based oils, and fluorohydrocarbon-based oils.
Application of the electrorheological fluid to engine mounts, shock absorbers, clutches, etc. can be expected.
However, any of the conventionally known electrorheological fluids are of disperse system, they cannot get rid of the problem that the dispersion particles are to separate and precipitate, which has been a main obstacle to practical application of the electrorheological fluid.
Attempts have been made to develop a homogeneous electrorheological fluid. For example, an electrorheological fluid containing low-molecular liquid crystals, such as methoxybenzylidenebutylaniline, has been studied (see Japanese Journal of Applied Physics, Vol. 17, p. 1525 (1978)), but is still unsuitable for practical use because of its very small electrorheological effect.
JP-B-42-11315 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses that a uniform solution containing high-molecular liquid crystals exhibits an electrorheological effect. It has been reported that a great electrorheological effect is obtained from a solution of poly(.gamma.-benzyl L-glutamate), which is known as a lyotropic liquid crystalline polymer, in a low-boiling polar solvent, such as dioxane, tetrahydrofuran or cresol, or a low-boiling chlorine-containing solvent, such as methylene chloride or chloroform, as disclosed in JP-A-4-191511, JP-A-4-266997, and Dai-16kai Ekisyo Toronkai Yokoshu, p. 82 (1990). Further, JP-A-6-330068 and JP-A-7-238290 describe that a great electrorheological effect is obtained from an .alpha.-methylphthalene solution of poly(.gamma.-benzyl L-glutamate-co-.gamma.-dodecyl L-glutamate) having improved solubility in an aromatic solvent.
The above-described electrorheological fluid using poly(.gamma.-benzyl L-glutamate) has a homogeneous system and can therefore be freed of precipitation of particles and also exhibits a relatively great electrorheological effect. Solvents proposed for dissolving poly(.gamma.-benzyl L-glutamate) include low-boiling polar solvents and low-boiling chlorine-containing solvents. However, these electrorheological fluids are still difficult to put into practical use due to different problems arising from the solvent, such as an electrical current readily runs therethrough; the electrodes are susceptible to corrosion; the solvent readily evaporates; the solvent has high toxicity. In addition, as the molecular weight of the poly(.gamma.-benzyl L-glutamate-co-.gamma.-dodecyl L-glutamate) increases, it becomes less soluble in an aromatic solvent, and the resulting solution has an increased viscosity before application of an electric field, making the difference in torque induced on application of an electric field smaller.